1. Field of the Invention
The present invention relates to an item of electrical apparatus, and in particular to apparatus for converting the supply voltage of a DC power supply.
2. Summary of the Prior Art
Recent years have seen the emergence and development of a wide range of electronic accessories for motor vehicles, motor boats and other large pieces of equipment. Among such electrical accessories are lights, heating units, and more recently of course increasingly sophisticated telecommunications devices. Rather than carry their own source of electrical power, many accessories are intended to draw energy from the battery power source of the larger pieces of equipment, and are therefore designed to be compatible with the 12 volt batteries which are now standard in motor cars. The optimum input voltage of many electronic accessories is in fact 13.8 volts.
Unfortunately, the DC supply format used in other industrial, military, commercial, aviation, maritime and other applications differs considerably. Large vehicles, for example, require electrical power to be carried over comparatively longer lengths of cable with, in addition, an increased number of devices using the DC supply.
Therefore, if the DC supply is doubled in voltage from the nominal 12 volts to a nominal 24 volts the current demand is halved although the overall power available would be unchanged.
For example, large commercial or heavy vehicles typically use the higher DC voltage format centred around a nominal 24 volts.
There is therefore a requirement for converters capable of receiving the output of these higher DC voltage formats and supplying current in an acceptable form to 12 volt format electric accessories, that is to say a converter capable for example, of providing a constant supply of 13.8 volts from a varying supply of between 23.3 volts and 27.6 volts.
It should be appreciated that such a converter may have to deliver a power supply of several watts, tens of watts or even hundreds of watts, and that in this context problems are encountered which have no counterpart in microelectronic power conversion systems. For example, U.S. Pat. No. 4,827,205 discloses an on-chip 10 volt voltage supply in which current is delivered through a 10k resistor, which limits the power delivery to be of the order of milli-watts. In such a context conversion efficiency is unimportant and heat generation causes no significant problems.
An early generation of DC power converters, often misnamed "Droppers", were based upon linear converters, which is to say devices which step-down and regulate a voltage supply principally using transistor technology. It was perceived, however, that such devices perform their tasks with unacceptably low power conversion efficiency. Furthermore, no design of linear converter was found which could provide an output voltage with sufficient stability, particularly when the current demand at the output increased to any significant degree.
Many devices used as accessories in vehicles, boats, the aviation industry or other equipment; require a reasonably smooth and stable DC supply voltage.
Recent developments in DC power converters have therefore concentrated on methods of DC power conversion in which a DC supply powers an oscillator circuit, often housed under the dashboard of the lorry, for generating an oscillating voltage across the terminals of a stepdown transformer. The output of the transformer is then rectified, smoothed and regulated to provide the desired supply, usually nominally 12 volts. Surprisingly, progressive refinements of this method have resulted in devices of up to 75% efficiency, and such systems are very widely employed.
The present inventor has found, however, that oscillation based power converters suffer from at least two serious disadvantages.
A first disadvantage of many switched-mode (oscillation) based converters is that their circuitry is all too likely to be damaged by the heat generated within them when the converter is abused, for example by direct electrical connection of its output terminals. In practice over the life of the converters operatives tend to replace any safety fuses (or fuses supplied with the converter) with incorrect fuses or, worse, by-pass them entirely.
This leads to significant fire hazards.
Secondly, they generate by their nature powerful electromagnetic radiation, often referred to as radio frequency interference, which is often radiated in a manner that affects electrical, electronic and more often communications equipment within the local area of the converter.
This is a widespread occurrence and, although many devices are claimed to have adequate filtering within their design, this problem occurs continually.
This problem is potentially more serious when the radiation affects users of devices and/or communications equipment completely remote and both unattached and unconnected to the converter mounted on the vehicle or equipment in question.
In many instances the user of the conversion device has no knowledge that it may be causing interference externally to other services.